It’s been 20 years since the Vaccines for Children (VFC) program provided free immunizations to all US youngsters. How well has the program done?

In the latest report card on childhood vaccination rates, the Centers for Disease Control says national immunization rates are close to or above 90% for diseases such as measles, mumps, rubella, polio, hepatitis B and chicken pox. Only 0.8% of U.S. children received no vaccines during the survey period, which included youngsters born between 2009 and May 2011.

“This year’s report card is very good, and the high coverage we see is why most vaccine preventable diseases are at record low levels,” Dr. Anne Schuchat, director of the CDC’s National Center for Immunization and Respiratory Diseases told reporter during a teleconference.

But the numbers hide the fact that this year, unimmunized children contributed to the largest outbreaks of measles in the U.S. since the disease was eliminated in 2000. New York city, North Carolina and Texas experienced outbreaks that accounted for most of the 159 cases of the disease reported so far; generally about 50 to 60 cases are reported annually.

“The thing about measles is that you don’t have to be closely connected to an infected person to develop measles,” Schuchat said. “Someone can bring measles into an arena and anybody who is not vaccinated is going to get that virus. It’s very, very transmissible.”

Most of the measles cases in the U.S. are imported from other countries, said Schuchat, which wouldn’t be a problem if more people were vaccinated against the disease. After confusing and unsubstantiated reports circulated in recent years about the dangers associated with the measles, mumps and rubella vaccine in particular – including rumors that it contributed to autism – many parents decided not to vaccinate their children. Of the measles cases in the U.S. so far this year, 82% were unvaccinated and 9% had unknown vaccination status.

Schuchat said vaccination against diseases like measles is important not just to protect those who get the shots but to help those who can’t receive the immunizations – such as infants younger than 12 months and those with compromised immune systems. The more people who are vaccinated, the stronger the effect of herd immunity, which acts as a biological shield against communicable diseases for entire populations.

Addressing why parents aren’t vaccinating their kids could improve immunization rates even more, said Schuchat, and may help to reduce or eliminate outbreaks of disease such as measles altogether.